At the time of the birth of general relativity (GR), experimental
confirmation was almost a side issue. Einstein did calculate
observable effects of general relativity, such as the perihelion
advance of Mercury, which he knew to be an unsolved problem, and
the deflection of light, which was subsequently verified, but
compared to the inner consistency and elegance of the theory, he
regarded such empirical questions as almost peripheral. But
today, experimental gravitation is a major component of the
field, characterized by continuing efforts to test the theory's
predictions, to search for gravitational imprints of high-energy
particle interactions, and to detect gravitational waves from
astronomical sources.

The modern history of experimental relativity can be divided
roughly into four periods: Genesis, Hibernation, a Golden Era,
and the Quest for Strong Gravity. The Genesis (1887-1919)
comprises the period of the two great experiments which were the
foundation of relativistic physics - the Michelson-Morley
experiment and the Eötvös experiment - and the two immediate
confirmations of GR - the deflection of light and the perihelion
advance of Mercury. Following this was a period of Hibernation
(1920-1960) during which theoretical work temporarily outstripped
technology and experimental possibilities, and, as a consequence,
the field stagnated and was relegated to the backwaters of
physics and astronomy.

But beginning around 1960, astronomical discoveries (quasars,
pulsars, cosmic background radiation) and new experiments pushed
GR to the forefront. Experimental gravitation experienced a
Golden Era (1960-1980) during which a systematic, world-wide
effort took place to understand the observable predictions of GR,
to compare and contrast them with the predictions of alternative
theories of gravity, and to perform new experiments to test them.
The period began with an experiment to confirm the gravitational
frequency shift of light (1960) and ended with the reported
decrease in the orbital period of the Hulse-Taylor binary pulsar
at a rate consistent with the general relativity prediction of
gravity wave energy loss (1979). The results all supported GR,
and most alternative theories of gravity fell by the wayside (for
a popular review, see [148]).

Since 1980, the field has entered what might be termed a Quest
for Strong Gravity. Many of the remaining interesting weak-field
predictions of the theory are extremely small and difficult to
check, in some cases requiring further technological development
to bring them into detectable range. The sense of a systematic
assault on the weak-field predictions of GR has been supplanted
to some extent by an opportunistic approach in which novel and
unexpected (and sometimes inexpensive) tests of gravity have
arisen from new theoretical ideas or experimental techniques,
often from unlikely sources. Examples include the use of
laser-cooled atom and ion traps to perform ultra-precise tests of
special relativity; the proposal of a ``fifth'' force, which led
to a host of new tests of the weak equivalence principle; and
recent ideas of large extra dimensions, which have motived new
tests of the inverse square law of gravity at sub-millimeter
scales. Several major ongoing efforts also continue, principally
the Stanford Gyroscope experiment, known as Gravity Probe-B.

Instead, much of the focus has shifted to experiments which
can probe the effects of strong gravitational fields. The
principal figure of merit that distinguishes strong from weak
gravity is the quantity
, where
G
is the Newtonian gravitational constant,
M
is the characteristic mass scale of the phenomenon,
R
is the characteristic distance scale, and
c
is the speed of light. Near the event horizon of a non-rotating
black hole, or for the expanding observable universe,
; for neutron stars,
. These are the regimes of strong gravity. For the solar system
; this is the regime of weak gravity. At one extreme are the
strong gravitational fields associated with Planck-scale physics.
Will unification of the forces, or quantization of gravity at
this scale leave observable effects accessible by experiment?
Dramatically improved tests of the equivalence principle or of
the inverse square law are being designed, to search for or bound
the imprinted effects of Planck-scale phenomena. At the other
extreme are the strong fields associated with compact objects
such as black holes or neutron stars. Astrophysical observations
and gravitational wave detectors are being planned to explore and
test GR in the strong-field, highly-dynamical regime associated
with the formation and dynamics of these objects.

In this Living Review, we shall survey the theoretical
frameworks for studying experimental gravitation, summarize the
current status of experiments, and attempt to chart the future of
the subject. We shall not provide complete references to early
work done in this field but instead will refer the reader to the
appropriate review articles and monographs, specifically to
Theory and Experiment in Gravitational Physics
[147], hereafter referred to as TEGP. Additional recent reviews in
this subject are [142,
145,
150,
139,
37,
117]. References to TEGP will be by chapter or section, e.g. ``TEGP
8.9 [147]''.